The National Academy of Engineering in Washington, D.C., once asked its members to pick the greatest engineering achievement ever. Their choice? The electrification of the country through what's known as "the grid." Ernest Moniz, director of the Energy Institute at MIT, says they were right on the money.

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"That reflects what an amazing machine this is, spread out geographically, always having to balance demand and supply because electricity is not stored," he says.

Every day, with the flick of a switch, millions of Americans tap into the electricity grid. It's a web of power stations, transformers and transmission lines that span the continent, distributing electricity like veins and arteries distribute blood.

Electricity has to keep flowing all the time. Grid operators constantly match what power plants are producing with what people and their TVs, microwaves and air conditioners need. It's the world's biggest balancing act.

Predicting The Unpredictable

That's doable largely because big power plants run almost constantly and produce a predictable amount of electricity.

So what happens when you add in unpredictable sources of electricity, like wind or solar power?

"The operator does not have control of when to turn it on and off," Moniz says. "It's a new challenge that we just have to meet, and we're not doing it at anything like the pace that I think we need."

That's the conclusion of a study that Moniz's group at MIT is issuing Monday. It's all about how the grid must change to handle the fickle flow of electrons from renewable energy.